Removal of SO2 from gases

ABSTRACT

Sulfur dioxide is separated from an aqueous solution containing the same by subjecting the stream to electrodialytic water splitting. In particular a novel method for removing SO 2  from dilute gas streams by means of alkaline solution scrubbing, regeneration of the scrubbing solution and liberation of concentrated SO 2  effected by means of a two-compartment membrane water splitter is provided. Optionally, waste sulfate produced in the process may be converted to sulfuric acid in a separate membrane water splitter or otherwise processed by conventional means.

BACKGROUND OF THE INVENTION

Environmental considerations preclude burning of fuels with high sulfurcontent, since the consequent production of sulfur dioxide presents aserious pollution problem. To overcome this, the sulfur in the fuel mustbe removed prior to, during or after combustion. For users of largequantities of fuel, such as utilities, removal of sulfur aftercombustion generally has been recognized as most feasible. In thisregard, considerable research has been done and a number of processesfor removal of sulfur dioxide from the combustion gases have beendeveloped. Among these are lime and limestone scrubbing, magnesium oxidescrubbing, sodium scrubbing with thermal regeneration, e.g. see U.S.Pats. Nos. 3,477,815 and 3,485,581, sodium scrubbing with electrolyticregeneration, e.g. see U.S. Pat. No. 3,475,122, citrate process (U.S.Bureau of Mines (Report of Investigations 7774, (1973)), phosphateprocess (Stauffer Chemical (Chem. Eng. 81, July 8, pp. 46-47 (1974), thedouble alkali process, the catalytic oxidation process to make sulfuricacid. A review of these and other processes has been made by A. V. Slackof Noyes Data Corp. ("Sulfur Dioxide Removal from Waste Gases" --Pollution Control Review No. 4 (1971) by A. V. Slack; Noyes Data Corp.;Park Ridge, N.J. 07656).

Many considerations enter into the decision of which type of processmight be best. Among these are capital and operating costs, reliability,flexibility in operation, production of useful sulfur products, disposalof byproducts, etc.

SUMMARY OF THE INVENTION

This invention is concerned with a new method for recovery of sulfurdioxide from gases containing the same. It relates specifically toselective removal and recovery of sulfur dioxide from a lean gas streamthat may contain other acidic gases.

Included among the objects and advantages of the invention are thefollowing:

1. To provide a process that removes SO₂ from gas at high efficienciessimultaneously with removal of dust, heat and/or flyash particlescontained in the gas. 2. To recover the SO₂ in a commercially useful,saleable or otherwise disposable form. 3. To provide complete orsubstantially complete recovery of the absorbent at moderatetemperatures (from 15° C. to 80° C.) without the use of substantialquantities of external heat or addition of processing chemicals. 4. Toprovide an efficient method for concentrating a relatively dilute streamof SO₂. 5. To provide a process that economically regenerates chemicals,substantially through movement of ions which are thermodynamically moreeasily moved when compared to use of phase change or use of electrodereactions.

Other objects and advantages will be apparent to those skilled in theart from the details that follow.

The invention involves scrubbing the flue gas from a furnace or boilerwith an aqueous alkaline solution, e.g.: Na₂ SO₃, NaOH, a combination ofboth or any other relatively basic solution. The treated scrubbersolution is divided and then treated in a membrane water splitterconsisting of cation exchange and bipolar membranes. This operationproduces a basic solution which is recycled to the scrubber and an SO₂containing solution. Since the solubility of SO₂ in water is low atelevated temperature, the SO₂ may be removed from the solution byheating and/or blowing air through it. The sulfate formed in theabsorber by oxidation of SO₂ by O₂, may be removed in a variety of ways,e.g. by further water splitting in a two-compartment cell and liming ofthe resultant NaHSO₄ to precipitate CaSO₄ or in a conventionalthree-compartment membrane water splitter to form H₂ SO₄ which may beconcentrated or limed, or by recovery of Na₂ SO₄ by evaporation of theSO₂ -free stream from the stripping operation, or by recovery of Na₂ SO₄as Glauber's salt by crystallization.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a flow sheet of the system for recovery of SO₂ in accordancewith the invention;

FIG. 2 is a diagrammatic illustration of a membrane stack which may beemployed for two compartment water splitting in accordance with theinvention;

FIG. 3 is a fragmentary diagram of a unit cell from a conventional watersplitter system;

FIG. 4 is a fragmentary diagram of a unit cell for regenerating acid andbase from weak base scrub liquors.

FIG. 5 is a part of a flow sheet of an alternate embodiment employingchemical conversion for purging sulfate ion from the system inaccordance with the invention;

FIG. 6 is a flow sheet for another method for purging sulfate from thesystem employing evaporation; and

FIG. 7 is a flow sheet of still another embodiment for removal ofsulfate which involves cooling a portion of the stripper bottoms andcrystallizing therefrom the sulfate salt.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The process may be better understood by reference to the drawing whereinreference numerals accompanied by lower case letters generally refer tosimilarly functioning elements or alternatives in other figures of thedrawing. In FIG. 1 the SO₂ -rich gas enters the absorber 1 from a stream5 where it reacts with the basic medium to form a bisulfite, e.g. NaOHto form NaHSO₃. In the absorber, oxidation converts some of the sulfurvalues to Na₂ SO₄. While sodium is used in this description it isunderstood that various other cations forming soluble sulfites which areknown to those skilled in the art also may be used, e.g., K⁺, Li⁺,quaternary ammonium ions such as tetramethyl ammonium, methylpyridinium, dimethyl piperidium, hydroxyethyldimethyl ammonium andsulfonium ions such as trimethyl sulfonium. Amines which will beprotonated by SO₂ in water, e.g., ammonia, methylamine, pyridine,dimethylamine and trimethylamine may also be used as the scrubbingmedium. Anions other than OH⁻ may be used to effect scrubbing, such asphosphate, sulfite or carboxylates, e.g. acetate, citrate or oxalate.The invention may be practiced whenever the conversion of a basiccompound to a substantially soluble more acidic compound by absorptionof SO₂ takes place in the absorber, since this absorption can befollowed by regeneration of the basic compound and liberation of SO₂ bythe water splitter as will be described. The absorber used in such anapplication may be one of a number of types that are well known in theliterature. Specifically, a venturi absorber, a plate absorber or apacked tower may be used. The particulates in the incoming gas may beremoved in a separate step prior to absorption (as in this embodiment)or the particulate removal may be combined with the absorption step. Thetemperature of absorption can be between 80° F. and 180° F., preferably,the temperature is between 90° F. and 155° F. The ion concentrations ofNa₂ SO₃, NaHSO₃, NaOH, Na₂ SO₄, NaHSO₄ in the scrubbing solutions canvary over a wide range and are limited, in theory, only by theirsolubilities. The depleted gas stream exits the scrubber as stream 20and presents no significant pollution problem because most of the SO₂has been removed. The exact composition of stream 6 which contains theabsorbed sulfur values will depend on the incoming absorption liquorcomposition, the design of the absorber, the degree of SO₂ removalsought, and the amount of sulfur value converted by oxidation tosulfate.

The reactions occuring in the absorber 1 can be summarized by thefollowing equations:

for NaOH scrubbing --

    SO.sub.2 + 2NaOH → Na.sub.2 SO.sub.3 + H.sub.2 O

    na.sub.2 SO.sub.3 + SO.sub.2 + H.sub.2 O → 2NaHSO.sub.3

    na.sub.2 SO.sub.3 + 1/2 O.sub.2 → Na.sub.2 SO.sub.4 (side reaction)

for ammonia scrubbing --

    2NH.sub.3 + SO.sub.2 + H.sub.2 O → (NH.sub.4).sub.2 SO.sub.3

    (nh.sub.4).sub.2 so.sub.3 + so.sub.2 + h.sub.2 o → 2nh.sub.4 hso.sub.3

    2nh.sub.3 + so.sub.2 + 1/2 o.sub.2 + h.sub.2 o → (nh.sub.4).sub.2 so.sub.4 (side reaction)

for acetate scrubbing --

    NaOAc + SO.sub.2 + H.sub.2 O → NaHSO.sub.3 + HoAc

    2NaOAc + SO.sub.2 + 1/2 O.sub.2 → Na.sub.2 SO.sub.4 + 2HoAc (side reaction)

Similar equations can be written for other scrubbing media. The sulfurvalue rich stream 6 from the absorber is split into two streams whichfeed a two-compartment water splitter, 2. Details of the operation ofthis water splitter will be provided in conjunction with the descriptionaccompanying FIG. 2.

The two-compartment water splitter 2 incorporates a plurality of cationand bipolar ion exchange membranes arranged in an alternating fashionbetween two electrodes thus forming an electrodialysis stack. Theconstruction of electrodialysis stacks is well known and, for example,units are available commercially from Asahi Glass Co., 1-2, Marunochi2-chome, Chiyoda-ku, Tokyo, Japan; Ionics, Inc., Watertown,Massachusetts and other commercial sources. In general, stacks which aresuitable for electrolyte concentration such as the Asahi Model CU-IV,may be used for the water splitter. However, the membrane used thereinmust be of a kind adaptable to water splitting. While the use of bipolarmembranes is to be preferred because of the simplicity of equipment, thewater splitting operation may be carried out by using a thirdcompartment containing anions or cations incapable of passing throughthe anion and cation membranes on either side of the compartment asdescribed, for example, in U.S. Pats. Nos. 3,704,218 and 3,705,846. Thisarrangement when used for water splitting operates by the sameprinciples as the bipolar membrane. The two compartment water splitteror its equivalent converts water into hydrogen and hydroxyl ion. Thewater splitter employs suitable bipolar membranes, that can be of thetype described, for example, in U.S. Pat. No. 2,829,095, which hasreference to water splitting generally, or any other type whicheffectively converts water into hydrogen and hydroxyl ions.

The operation of the water splitter which is further described byreference to FIG. 2, is essentially as follows: A direct current passesfrom the anode 26 to the cathode 25. Stream 8a which contains NaHSO₃ andNa₂ SO₄ from the absorber is fed to the compartments labeled A. Incompartments A, hydrogen ion, H⁺, from the bipolar membranes 28 isintroduced. At the same time, cations migrate to the B compartmentsthrough the cation membranes 27. Since SO₂ is a weak acid, theconcentration of H⁺ ion in the A compartments is low until all of theHSO₃ ⁻ ion is converted to SO₂. This promotes the effective transport ofNa⁺ ion (relative to H⁺ ion) across the cation membrane. The presence ofsome Na₂ SO₄ in stream 8 is a benefit to the effectiveness since itraises the ratio of Na⁺ /H⁺ ions in the A compartments. The otherportion 7a of the stream from the absorber (shown as numeral 7 inFIG. 1) is fed to the B compartments of the two compartment watersplitter which is illustrated in detail in FIG. 2. The OH⁻ ion from thebipolar membrane reacts with any HSO₃ ⁻ ion in the stream to form SO₃ ⁼ion with a substantially neutral charge being maintained by the incomingflux of Na⁺ ions from the A compartments. When all of the HSO₃ ⁻ ion hasreacted, NaOH will be produced. The composition of the base stream will,therefore, depend on the relative amounts of stream 6 from the scrubberintroduced into the A and B compartments as stream 8a and 7arespectively, and may be essentially all NaOH, Na₂ SO₃, mixtures of NaOHand Na₂ SO₃ or NaHSO₃ /Na₂ SO₃ mixtures. The conversion of the solutionmay be made in a single pass through the stack or by a feed and bleedapportionment method or by passing the solutions through a series ofstacks so that the solubility of SO₂ in water is not exceeded and no gasis formed inside the water splitting stack. The operation of the watersplitter with the pressure on the solutions greater than atmospheric mayalso help prevent the formation of SO₂ gas inside the stack.

The reactions in the water splitter are schematically represented by thefollowing equations:

Acid Compartment:

for NaOH/Na₂ SO₃ scrubbing --

    NaHSO.sub.3 + H.sup.+ - Na.sup.+ → H.sub.2 SO.sub.3

    h.sub.2 so.sub.3 ⃡ h.sub.2 o + so.sub.2

for ammonia scrubbing --

    NH.sub.4 HSO.sub.3 + H.sup.+ - NH.sub.4.sup.+ → H.sub.2 SO.sub.3

    h.sub.2 so.sub.3 ⃡ h.sub.2 o + so.sub.2

for acetate scrubbing --

    NaHSO.sub.3 + HOAc + H.sup.+ - Na.sup.+ → H.sub.2 SO.sub.3 + HOAc

    H.sub.2 SO.sub.3 ⃡ H.sub.2 O + SO.sub.2

base Compartment:

for NaOH/Na₂ SO₃ scrubbing --

    NaHSO.sub.3 + Na.sup.+ + OH.sup.- → Na.sub.2 SO.sub.3 + H.sub.2 O

    na.sup.+ + OH.sup.- → NaOH

for ammonia scrubbing --

    NH.sub.4 HSO.sub.3 + NH.sub.4.sup.+ + OH.sup.- → (NH.sub.4).sub.2 SO.sub.3 + H.sub.2 0

    nh.sub.4.sup.+ + oh.sup.- → nh.sub.4 oh

for acetate scrubbing --

    NaHSO.sub.3 + HOAc + Na.sup.+ + OH.sup.- → NaHSO.sub.3 + NaOAc + H.sub.2 O

    naHSO.sub.3 + NaOAc + HO + Na.sup.+ + OH.sup.- → Na.sub.2 SO.sub.3 + NaOAc + 2H.sub.2 O

    na.sup.+ + OH.sup.- → NaOH

net Reactions:

for NaOH/Na₂ SO₃ scrubbing --

    2NaHSO.sub.3 → H.sub.2 O + SO.sub.2 + Na.sub.2 SO.sub.3

    naHSO.sub.3 → NaOH + SO.sub.2

for ammonia scrubbing --

    2NH.sub.4 HSO.sub.3 → H.sub.2 O + SO.sub.2 + (NH.sub.4).sub.2 SO.sub.3

    nh.sub.4 hso.sub.3 → so.sub.2 + nh.sub.4 oh

for acetate scrubbing --

    3NaHSO.sub.3 + HOAc → NaOAc + Na.sub.2 SO.sub.3 + 2SO.sub.2 + 2H.sub.2 O

    naHSO.sub.3 → NaOH + SO.sub.2

in the above equations, only the decompositions of bisulfite in the feedstream 6 has been shown. In practice, the feed to the water splittermight consist of a mixture of sulfite and bisulfite. The sulfite willundergo water splitting as illustrated by the reactions below.

Acid Compartment:

    Na.sub.2 SO.sub.3 + 2H.sup.+ - 2Na.sup.+ → H.sub.2 SO.sub.3

    h.sub.2 so.sub.3 ⃡ h.sub.2 o + so.sub.2

and similar equations for other systems.

Base Compartment:

    2Na.sup.+ + 2OH.sup.- → 2NaOH

net Reaction:

    Na.sub.2 SO.sub.3 + H.sub.2 O → 2NaOH + SO.sub.2

in a like manner similar equations can be written for other scrubbingsystems.

A significant advantage is provided in using a membrane water splitterfor regenerating the spent sulfite liquor because the process involvesno phase change, except for the evolution of SO₂ from H₂ SO₃ solutionsand requires only a small energy input. Furthermore, the process of theinvention has a high efficiency and can be operated at any convenienttemperature, e.g. within the broad range of about 50° F. to about 170°F. but more practically and preferably between about 70° F. and 130° F.

Electrolysis is not as effective as water splitting for this process.The main difference between electrolysis to produce acid and base fromsalt and water and electrodialytic water splitting by membranes to carryout the same process is that electrolysis generates H⁺ and OH⁻ ions atthe electrodes only and, at the same time, generates H₂ and O₂ (or otherelectrode oxidation and reduction products). For electrolysis, eachequivalent of H⁺ and OH⁻ generated will result in an equivalent amountof H₂ and O₂ (or other oxidation and reduction products) being produced.

Water splitting, on the other hand, generates, H⁺ and OH⁻ ions from eachof the several bipolar membranes between the electrodes without formingH₂ and O₂ (except for the relatively limited quantity of H₂ and O₂ atthe electrodes which are the only places where oxidation-reduction istaking place). Therefore, in water splitting oxidation-reductionproducts are formed in only small amounts relative to the total amountof H⁺ and OH⁻ formed at the bipolar membranes. In general, the processof electrolysis requires more energy than does water splitting since theenergy required to produce H₂ and O₂ from water in electrolysis must besupplied in addition to the energy needed to produce H⁺ and OH⁻ fromwater. In addition to the higher energy consumption for electrolysis, inthe system of the present invention, the electrolysis of sulfite andbisulfite solutions is known to produce mostly undesirable products suchas dithionate and sulfate at the anode, see for example the Bureau ofMines, Information Circular 7836 (1958), p 48.

As shown in FIG. 1, the processed solution 10 from the B compartments ofstack 2 is recycled to the absorber 1. The solution from the Acompartments (FIG. 2) exits the water splitter as stream 9. Stream 9consisting primarily of Na₂ SO₄, SO₂ and water is introduced intostripper 3 where it is heated and the dissolved SO₂ liberated andcollected as stream 11. The concentrated SO₂ in stream 11 may beconverted to sulfur or sulfuric acid or compressed and recovered asliquid SO₂ by known technology.

SO₂ may also be recovered from stream 9 by blowing air through thesolution, eliminating the need for heating the solution. The mixture ofSO₂, N₂, O₂ obtained from this operation would be suitable for themanufacture of sulfuric acid.

Another method of obtaining SO₂ from stream 9, FIG. 1, would be topressurize the stream and obtain liquid SO₂ as the overhead product froma distillation column. Alternatively, the fractionation may be carriedout under reduced pressure at ambient temperatures and the evolved SO₂liquefied by compression. It is apparent that various other methods ofremoving SO₂ from the SO₂ -rich stream 9 known to those skilled in theart may also be used.

Stream 12 from the bottom of stripper 3 consists essentially of aqueousNa₂ SO₄. In order to purge sulfate formed by oxidation in the process,stream 12 may be treated in a three-compartment water splitter of theconventional type, for example, apparatus of the kind described in U.S.Pat. No. 2,829,095 or in U.S. Pats. Nos. 3,704,218 and 3,705,846 anddepicted diagrammatically as 4 in FIG. 1. A unit cell from the watersplitter 4 is illustrated in greater detail in FIG. 3. A series ofmembranes, as seen by reference to FIG. 3, cation 34, bipolar 33, anion32, are placed between an anode 31 and a cathode 30 forming a series ofthree chambers designated S, B and A. The Na₂ SO₄ stream 12b is fed tothe S chambers where under the action of a direct current, itsconcentration is reduced by migration of Na⁺ ions to the B compartmentsand by migration of SO₄ ⁼ ions to the A compartments. The depleted Na₂SO₄ exiting from the S compartments of water splitter 4, stream 13 (seeFIG. 1) may be introduced into the B compartments to receive NaOH whichexits from water splitter 4 as stream 14 and is returned to theabsorber 1. Into the water splitter 4, as shown in FIG. 1, water (stream17) and some recycled acid (stream 19) are introduced as stream 18 intothe acid compartments of the water splitter 4 where the concentration ofH₂ SO₄ is increased and leaves the water splitter as stream 21. Aportion (Stream 16) of stream 21 can then be concentrated further,disposed of in a manner known in the art or utilized as is. Only enoughNaOH and H₂ SO₄ are produced in the water splitter 4 to remove the SO₄ ⁼ions formed by oxidation and maintain a constant amount of recyclesulfate to the absorber.

The net reaction in the three-compartment water splitter is:

    ______________________________________                                        Na.sub.2 SO.sub.4 +                                                                     2H.sup.+ + 20H.sup.- → 2NaOH +                                                              H.sub.2 SO.sub.4                               (salt     (from        (base       (acid                                      compart-  bipolar      compart-    compart-                                   ment S)   membrane)    ment B)     ment A)                                    ______________________________________                                    

If a weak base such as ammonia is used in scrubbing, the removal ofsulfate may be achieved more simply. The three-compartment watersplitter can be replaced by a two-compartment water splitter, consistingof bipolar and anion membranes. A unit cell from such a water splitteris illustrated in FIG. 4. A series of membranes, bipolar 33c and anion32c are placed between an anode 31c and a cathode 30c forming a seriesof two chambers designated B' and A'. A stream of (NH₄)₂ SO₄ from thestripper 3 in FIG. 1 is fed to the base (B') compartments as stream 12c.Because of the relative abundance of sulfate ions, SO₄ ⁼ is transferredacross the anion membranes, in preference to the OH⁻ ions. In the A'compartments, these sulfate ions combine with the H⁺ ions generated bythe bipolar membranes, to yield H₂ SO₄. Stream 18c (18 in FIG. 1) isused to pick up the sulfuric acid. The more concentrated sulfuric acidexits this water splitter as stream 21c (21 in FIG. 1). The reactionsoccurring are:

    ______________________________________                                        Base Compartments (B')                                                        (NH.sub.4).sub.2 SO.sub.4 +                                                              OH.sup.-  →NH.sub.4 OH                                                              + SO.sub.4.sup.=                                                 (from bipolar                                                                               (goes through                                                   membrane)      anion membrane)                                     Acid Compartment (A')                                                         2H.sup.+ + SO.sub.4.sup.=  → H.sub.2 SO.sub.4                          Net Reaction                                                                  (NH.sub.4).sub.2 SO.sub.4 + 2H.sup.+  + 20H.sup.- → 2NH.sub.4 OH +     H.sub.2 SO.sub.4                                                              ______________________________________                                    

Another method of purging SO₄ ⁼ ion from the system is described byreference to FIG. 5. The Na₂ SO₄ solution from the stripper 3 (ofFIG. 1) shown in FIG. 4 as stream 12d (the whole or a portion of thestream 12 from FIG. 1) is fed to the acid side of a two-compartmentwater splitter 40. A conversion from Na₂ SO₄ to NaHSO₄ occurs in amanner analogous to the conversion of NaHSO₃ to H₂ SO₃ which occurs inthe water splitting operation described in conjunction with FIG. 2.Enough acidity is introduced into the Na₂ SO₄ fed into splitter 40 sothat in the subsequent reactions, the required amount of sulfate isremoved. Additional base is generated on the base side of the watersplitter. Stream 10d is used to pick up the base generated. This streamcan be a portion of stream 10 or 6 from FIG. 1.

The water splitter reactions are:

Acid Compartment

    Na.sub.2 SO.sub.4 + H.sup.+ - Na.sup.+ → NaHSO.sub.4

base Compartment

    Na.sup.+ + OH.sup.- → NaOH

    naHSO.sub.3 + NaOH → Na.sub.2 SO.sub.3 + H.sub.2 O

net Reactions

    Na.sub.2 SO.sub.4 + NaHSO.sub.3 → NaHSO.sub.4 + Na.sub.2 SO.sub.3

    na.sup.+ + OH.sup.- → NaOH

similar equations can be written for other scrubbing systems. The Na₂SO₄ /NaHSO₄ solution from the water splitter 40, stream 44, is reactedat 41. Lime or limestone is introduced in stream 45. The slurry, stream46, consisting of dissolved Na₂ SO₄, small amounts of dissolved Ca(OH)₂and a precipitate of CaSO₄ is filtered at 42 and the sulfate removed at47 from the system as solid CaSO₄ or CaSO₄.sup.. 2H₂ O. The sodiumsulfate stream 48 may be treated with Na₂ CO₃ or CO₂ to reduce theconcentration of soluble Ca⁺⁺ ion by precipitating CaCO₃ which can thenbe filtered or the stream 48 can be mixed with stream 49 from watersplitter 40 which results in the formation of a small amount of CaSO₃which is separated in filter 43. The CaSO₃ removed at 51 is disposed ofand the alkaline solution 52 is returned to the absorber 1 of FIG. 1.

Instead of a separate water splitter to convert Na₂ SO₄ to NaHSO₄ theconversion may be carried out in water splitter 2 (FIG. 1) beforeintroducing the stream 9 to the SO₂ stripper 3. In this case, stream 12will be a mixture of Na₂ SO₄ and NaHSO₄ and may be treated with limewithout further acidification by water splitting.

Another method for purging sulfate from the system is by evaporation ofthe Na₂ SO₄ solution from the stripper. This type of purging processwill be described in conjunction with FIG. 6. As shown in FIG. 6, theSO₂ -rich stream 9e from the water splitter 2e (the latter designated 2in FIG. 1) is introduced into the stripper 3e where SO₂ (11e) is removedby heating. The sodium sulfate solution withdrawn at 12e is thendivided. One part 69 is returned to the absorber (1 in FIG. 1); theother part of the divided stream 61 is concentrated in evaporator(s) 80;the slurry from the evaporator stream 63, is filtered in filter 81 andthe solid Na₂ SO₄ dried at 82 and recovered at 83 for sale or disposalwhile the saturated solution from the filter 81, stream 65, is returnedto the evaporator 80. Steam may be recovered from the evaporator asshown at 67 and may be used for heating in the stripper 3e. It may beadvantageous also in some cases to divide stream 9e (not shown in FIG.6) before introduction to the SO₂ stripper so that two strippers areemployed; one of which can be operated by direct steam injection and theother of which uses a heat exchanger and provides the Na₂ SO₄ stream tobe evaporated. This type of operation makes use of the efficient heatingmethod of direct steam injection without diluting with condensed steamthe stream which is to be evaporated.

When Na₂ SO₄ is produced, the Na⁺ ion and base values equivalent to theamount of SO₄ ⁼ produced are lost and must be made up. This may beeffected by the addition of Na₂ CO₃, NaHCO₃, Na₂ SO₃, NaHSO₃ or NaOH(stream 71) to stream 10e from the B compartments of the water splitter2e before returning to the absorber as stream 15e (15 in FIG. 1).

Alternatively, the make-up Na₂ SO₃, NaHSO₃, NaHCO₃, Na₂ CO₃ initiallycan be decomposed in a two compartment water splitter, therebygenerating NaOH and SO₂ (or CO₂). The base so generated can be fed asstream 71 to stream 10e. The SO₂ (or CO₂) generated can then berecovered or suitably disposed of.

Another method for purging sulfate from the system is by cooling aportion of the Na₂ SO₄ from the stripper, i.e. the stream shown at 12ein FIG. 6 and also as 12f in FIG. 7. This is described by reference toFIG. 7 wherein a portion 61f of the stripper bottoms, 12f, is chilledand the sodium sulfate crystallized out in a chiller crystallizer, 84.The mother liquor 86, after removing the sodium sulfate crystals asGlauber's salt at 87 is returned to SO₂ absorber (not shown) as stream88, along with stream 69f. The Glauber's salt that is crystallized outat 89 can subsequently be dried as shown at 90 and sold as sodiumsulfate. The sodium which is lost from the system must be made up as Na₂CO₃ NaHCO₃, Na₂ SO₃, NaHSO₃ or NaOH.

Additional modifications of the process will be apparent to thoseskilled in the art within the scope of the essence of the inventionherein set forth involving the liberation of SO₂ from the exhaustedscrubber solution and regeneration of fresh scrubber solution by twocompartment water splitting.

The invention will be further illustrated by reference to the followingexamples the details of which should not be construed as limiting theinvention except as may be required by the appended claims.

EXAMPLE 1

The efficiency of base and SO₂ production from solutions resemblingthose which would be obtained from the absorber were determined inlaboratory electrodialysis units. The units consisted of a platinumanode, a C-322* (AMF) cation membrane, another C-322 cation membrane, abipolar membrane with the anion permeable side facing the anode, anotherC-322 membrane and a platinum anode. The membranes were held in placeand separated by polyfluorocarbon cylinders of inside diameter 3.7 cmwhich contained ports for pumping the solutions to the 1.5 cm thicksolution compartments formed by the cylinder and the membranes. A cellwith five compartments was thereby formed, the compartments beinganolyte, acid, base, acid and catholyte. The solutions were pumped fromreservoirs, through the cell, and returned to the reservoir. The anolyteand catholyte were supplied from a common reservoir as were the acidcompartments. The reservoir supplying the central (base) compartment wascalibrated so that the volume of solution in this part of the systemcould be accurately determined.

The base compartment was charged with a solution made up by dissolving300 g Na₂ SO₃, 50 g Na₂ S₂ O₅, 57 g Na₂ SO₄ in 587 g of H₂ O. Analysisof this solution indicated that the initial base solution was 0.489MNaHSO₃. The acid compartment was charged with 500 ml of a solution0.366M in SO₂, 0.489M in NaHSO₃ and about 0.5M in Na₂ SO₄. Theelectrolyte solution was 1 liter of 0.5M Na₂ SO₄. A direct current of1.0 ampere was passed for a total of 10,000 sec. The volume of the basesolution increased from 129.5 ml initially to 139 ml. After the passageof current, the composition of the acid was 0.478 M SO₂, 0.336M NaHSO₃(no attempt was made to contain all of the SO₂ generated). Analysis ofthe base compartment indicated that no NaHSO₃ remained and that thesolution was 0.173M in NaOH. The overall efficiency for base production(i.e., loss of NaHSO₃ and gain of NaOH) was 84%.

EXAMPLE 2

A cell similar to that used in Example 1 was employed. The basecompartment was charged with 105 ml of 0.1027N KOH. The acidcompartments were charged with 1 liter of solution containing 0.4MKHSO₃, 0.3M H₂ SO₃ and 0.5M K₂ SO₄ (total acidity = 1.07N). Theelectrolyte compartments were charged with 1 liter of 0.25M K₂ SO₄. Adirect current of 1.0 ampere was passed for 6,000 sec. The volume of thebase solution increased to 108.4 ml. Analysis of the base showed thatthe solution was 0.540M KOH and 0.019M K₂ SO₃ so the current efficiencywas 76% for OH⁻ production. The total acidity of the acid compartmentincreased to 1.11N (no attempt was made to contain all the SO₂generated).

EXAMPLE 3

An electrodialysis unit similar to that described in Example 1 was usedexcept that the order of membranes (from the anode side) was C-322,bipolar, C-322, C-322. The compartments thus formed were anolyte, base,acid, base, catholyte. The acid compartment was fed from a reservoircalibrated so the volume of that portion of the system could beaccurately determined.

The acid compartment was charged with 170 ml of a solution 1.680M inNaHSO₃ and about 0.5M of Na₂ SO₄ and closed to minimize the loss of SO₂.The base compartment was charged with 483 ml. of 0.1012M NaOH. Thecurrent was 0.85 ampere for 12,000 sec. During the passage of current,the volume of the acid compartment was kept constant at 170 ml by addingH₂ O. After the passage of current, the acid compartment was 1.150MNaHSO₃ and 0.458M in SO₂. The volume of the base following the run was490 ml of solution which was 0.2530M in NaOH and 0.0093M in Na₂ SO₃. Theefficiency for SO₂ production, therefore, was 74% and for NaOH 71% basedon the current passed.

EXAMPLE 4

In an apparatus similar to that described for Example 3, the basecompartment was charged with 600 ml of 0.25M NaHSO₃ and the acidcompartment with 1M Na₂ SO₄. The current was commenced at 1.0 amp. Attime intervals shown in Table I, 2 ml samples were withdrawn and theconcentration of acid determined. From the concentration measurement andthe volume change, the current efficiency during the interval wasdetermined.

                  TABLE I                                                         ______________________________________                                        Time                      Acidity at                                                                             % current                                  Interval                  end of   efficiency for                             (sec.)  V.sub.o (ml)                                                                           V.sub.f (ml)                                                                           interval (M)                                                                           the interval                               ______________________________________                                          0-2000                                                                              150.5    147.1    .1388    99                                         2000-4000                                                                             145.1    143.5    .2652    86                                         4000-6000                                                                             141.5    140.5    .3922    85                                         6000-8000                                                                             138.5    137.5    .5114    77                                         8000-9200                                                                             135.5    134.5    .5826    73                                         ______________________________________                                    

The acidity of the base compartment decreased to about 0.1M during thepassage of current.

EXAMPLE 5

A typical mass flow for the streams in FIG. 1 is given in Table IIbelow. The mass flow given is provided to help clarify the operation ofthe process and does not necessarily reflect optimum or realizableconditions for the operation of the process.

                                      TABLE II                                    __________________________________________________________________________    Stream #                                                                           Na.sub.2 SO.sub.4                                                                  Na.sub.2 SO.sub.3                                                                  NaHSO.sub.3                                                                        NaOH                                                                              SO.sub.2                                                                          O.sub.2                                                                         H.sub.2 SO.sub.4                                                                   H.sub.2 O                                  __________________________________________________________________________     5*                                                                            6   4,262                                                                              1,261                                                                              18,733              79,755                                      7   2,244                                                                              664  9,859               41,975                                      8   2,018                                                                              597  8,874               37,780                                      9   2,018              5,765      33,000                                     10   2,244                                                                              12,605                   48,380                                     11                      5,765                                                 12   2,018                         33,000                                     13   596                           32,820                                     14   596            800            32,820                                     15   2,840                                                                              12,605    800            81,200                                     16                            981  8,830                                      17                                 9,009                                      18                            981  17,839                                     19                            981  8,830                                      20*                                                                           21                            1,962                                                                              17,660                                     __________________________________________________________________________     *It has been assumed that 6406 parts of SO.sub.2 are absorbed from gaseou     stream 5 and that 160 parts of O.sub.2 have been consumed.               

All other components in stream 5 exist in stream 20.

We claim:
 1. A method for removing SO₂ from a gas which comprises:(a)absorbing SO₂ from said gas by contact with a basic aqueous solutionthereby forming an aqueous SO₂ -containing salt solution in whichsoluble sulfites and bisulfites are present; (b) dividing said solutioninto two streams A and B; (c) subjecting said streams to electrodialyticwater splitting in a two-compartment water splitter comprised ofalternating cation and bipolar membranes wherein one of said streams isintroduced into the water splitter compartments between the cationmembranes and the anion sides of the bipolar membranes; (d) introducingthe other stream, A, into the compartments of said splitter between thecation sides of the bipolar membranes and the cation membranes; (e)passing a direct current through the water splitter thereby effectingthe acidification of the stream A and the basicification of the stream Band the transfer of cations from stream A to stream B and producing anaqueous SO₂ containing solution derived from stream A and a solutioncomprised of base capable of absorbing SO₂ derived from stream B.
 2. Theprocess of claim 1 in which the scrubbing solution contains as a sourceof supporting electrolyte, a salt selected from the group consisting ofsodium, potassium and ammonium.
 3. The process of claim 2 in which thesupporting electrolyte salt is a sulfate.
 4. The process of claim 1 inwhich the gas from which SO₂ is removed is the combustion gas from aboiler which burns a sulfur containing fuel.
 5. The process of claim 1in which the gas from which SO₂ is removed is the effluent from asulfuric acid plant.
 6. The process of claim 1 in which the gas fromwhich SO₂ is removed is the effluent from a smelter.
 7. The process ofclaim 1 in which the basic aqueous solution contains sodium ion.
 8. Theprocess of claim 1 in which the basic aqueous solution containspotassium ion.
 9. The process of claim 1 in which the basic aqueoussolution contains ammonium ion.
 10. The process of claim 1 comprisingthe further step of separating the SO₂ from the SO₂ containing solutionobtained from the water splitter by heating the effluent andvolatilizing the SO₂.
 11. The process of claim 1 comprising the furtherstep of separating the SO₂ from the SO₂ containing solution obtainedfrom the water splitter by passing air through the stream to remove theSO₂.
 12. The process of claim 1 comprising the further step ofseparating the SO₂ from the SO₂ containing solution obtained from thewater splitter by subjecting it to fractional distillation underpressure to recover liquid SO₂.
 13. The process of claim 1 wherein SO₂is recovered from the SO₂ containing solution obtained from the watersplitter by fractionation under subatmospheric pressure.
 14. The processof claim 1 in which a pressure greater than atmospheric is maintained inthe water splitter thereby enhancing the solubility of the SO₂.
 15. Theprocess of claim 10 in which sulfate produced in step (a) is purged fromthe water splitter effluent subsequent to removal of SO₂ by subjectingsaid effluent to water splitting in a three-compartment water splittercontaining cation, anion and bipolar membranes to generate H₂ SO₄ andbase.
 16. The process of claim 10 in which sulfate produced in step (a)is purged, in the case of a weak base scrubbing solution, by subjectingthe water splitter effluent subsequent to removal of SO₂, to watersplitting in a two-compartment water splitter containing alternatingbipolar and anion membrane to produce sulfuric acid and base streams.17. The process of claim 10 in which the sulfate produced in step (a)and present in the effluent from the water splitter after removal of SO₂is converted, by subjecting this effluent to water splitting in atwo-compartment water splitter containing alternating cation and bipolarmembranes, to bisulfate and base streams.
 18. The process of claim 17 inwhich the bisulfate stream is subsequently treated with a base, selectedfrom calcium and barium that forms an insoluble sulfate, to effect aconversion of the bisulfate to calcium sulfate and soluble sulfate andlessening the respective residual calcium or barium concentration insolution by further treatment of the solution with a carbonate orsulfite.
 19. The process of claim 10 in which the sulfate produced instep (a) and present in the effluent from the water splitter subsequentto removal of SO₂ is purged by subjecting said stream to cooling tocrystallize sulfate salt.
 20. The process of claim 1 wherein the productstream derived from the water splitter is purged of sulfate byevaporating at least a portion of the stream to recover sulfate salt.21. The process of claim 4 in which the water splitting is effecteduntil stream A contains HSO₄ ⁻, separating SO₂ from the water splitterproduct stream, treating the stream from which SO₂ has been removed withcalcium base to produce calcium sulfate and soluble sulfate andlessening the calcium concentration in solution by further treatment ofthe solution with a carbonate or sulfite.
 22. The process of claim 18 inwhich the soluble sulfate is selected from the group of Na₂ SO₄, K₂ SO₄,and (NH₄)₂ SO₄ and the base generated comprises a product selected fromthe group consisting of NaOH, KOH, NH₄ OH, a mixture of NaOH and Na₂SO₃, KOH and K₂ SO₃, NH₄ OH and (NH₄)₂ SO₃, Na₂ SO₃, K₂ SO₃ or (NH₄)₂SO₃, a mixture of Na₂ SO₃ and NaHSO₃, K₂ SO₃ and KHSO₃ or (NH₄)₂ SO₃ andNH₄ HSO₃ and the calcium base is lime or limestone.
 23. The process ofclaim 2 wherein the solution subjected to electrodialytic watersplitting contains at least 0.1 equivalents/liter of a salt which isincapable of generating SO₂ when subjected to said water splitting. 24.The process of claim 23 wherein the salt is an alkali metal sulfate orammonium sulfate.
 25. A process for removing SO₂ from a gas whichcomprises (a) absorbing SO₂ from said gas with a basic aqueous solutioncontaining cations which extracts the SO₂ from the gas and formssubstantially soluble sulfites and bisulfites, (b) dividing saidsolution and (c) generating SO₂ and an aqueous solution capable ofabsorbing SO₂ from the SO₂ -containing solution by subjecting saiddivided solution to electrodialytic water splitting.